1. Field of the Invetion
The present invention relates to a method of producing a very high velocity water jet, such as that used in cutting materials, and more particularly to a method of producing a water jet in a manner to increase the operating life time of a nozzle during which it is able to produce a coherent water jet.
2. Brief Description of the Prior Art
It has long been known that a high velocity water jet can be used to cut through a variety of materials. For example, U.S. Pat. No. 2,985,050, Schwacha, discloses a method of providing a very high velocity water jet (e.g. four thousand feet per second) to cut through even very hard materials, such as metal alloys. U.S. Pat. No. 3,532,014, Franz, discloses a similar method where a high velocity water jet (with a velocity between approximately twelve hundred and four thousand feet per second) is used to cut through soft material.
It has long been recognized that if the liquid jet is formed as a very coherent stream (i.e. with very little dispersion), the cutting capability of the jet is substantially enhanced. Thus, there have been various attempts to improve the coherency of a high velocity water jet. One approach is to attempt to improve the flow by means of particular nozzle configurations. Two such examples of this approach are shown in U.S. Pat. No. 3,750,961, Franz, and U.S. Pat. No. 3,756,106, Chadwick et al, each of which provides a nozzle having a convergent passageway leading to the nozzle discharge opening.
Another prior art approach to improving the coherency of a high velocity liquid jet is disclosed in U.S. Pat. No. 3,524,367, Franz. The method disclosed in this patent involves dissolving long chain polymers in the working fluid (e.g. water) and then discharging the fluid through an orifice at very high velocity. It is stated in the patent that the liquid then exhibits a certain apparent viscosity at the very high shear rates encountered when the water with the polymer dissolved therein is discharged through a nozzle, which allegedly accounts for the improved coherency of the jet stream.
Another consideration in providing a high quality liquid jet is the effect of the deterioration of the operating components of the apparatus. Such apparatus comprises a high pressure intensifier to pressurize the working fluid in a chamber to a very high level (e.g. to as high as fifty thousand to one hundred thousand psi or higher), and a very small nozzle orifice (e.g. 0.001 to 0.040 inch) through which the working fluid is discharged to form the liquid jet. Quite commonly, the material which makes up the orifice is a very hard material, such as corundum crystal, formed within very close tolerances in a configuration with a minimum of irregularities which might disturb the flow. This is to minimize turbulence in the water passing from the nozzle orifice, since such turbulence would have the effect of causing unwanted dispersion in the jet stream. In the area closely adjacent to the nozzle orifice, the working fluid (e.g. water) accelerates at a very rapid rate to exit from the nozzle orifice as a very small stream travelling at a velocity as high as three thousand feet per second or higher. The area of the orifice is very critical in the formation of a coherent jet.
However, even when the orifice is made of a very hard material formed within very closed tolerances, over a period of time of possibly ten to twenty hours, the performance of the nozzle tends to deteriorate, which is evidenced by a quite noticeable dispersion of the water jet. This in turn requires the shutting down of the apparatus so that the crystal which forms the nozzle orifice can be replaced.